Gelidocalamus fengkaiensis (Poaceae: Bambusoideae), a new bamboo species from Guangdong, China, with an analysis of branch development in relation to flowering

Background Bamboos, widely distributed in temperate and tropical Asia, Africa and America, refer to a group of special plants in Poaceae, Bambusoideae. China is rich in bamboo species. However, due to a long flowering cycle, the flowering habit and the flowering structure of many bamboo species are still not well understood. Here, we report a new bamboo species from Guangdong, China and an analysis of its interesting branch development in relation to flowering. Results This species is similar to G. stellatus, the type species, but differs in the characteristics of its lemma and palea, mid-culm branch complement, and culm-sheath ligules. The initial branches at a culm node do not apically develop flowering structures during a flowering episode; instead, these form on what appears to be specialized flowering branches. Conclusions The results of morphological comparison support the recognition of Gelidocalamus fengkaiensis as a new species. And during a flowering episode, two branch types (‘foliage branch’ and ‘flowering branch’) can be distinguished in this species.

For Guangdong province, Lin (1988Lin ( , 1990Lin ( , 1992 published three Gelidocalamus bamboos, namely G. velutinus W. T. Lin, G. subsolidus W. T. Lin & Z. J. Feng and G. albopubescens W. T. Lin & Z. J. Feng. Then, Xia (2005) also recorded G. tessellatus in Guangdong province. Xia and Lin (2009) included a total of four species in their Flora of Guangdong account. However, G. albopubescens was considered as synonym of G. subsolidus by Liu et al. (2017), because he reckoned these two species share the same vegetative morphological features including similar microscopic features of the abaxial leaf epidermis ). Subsequently, Nie et al. (2018) reckoned that G. stellatus also occurs in Guangdong province.
During fieldwork in Qixingding Nature Reserve, Fengkai County, Guangdong province, a bamboo flowering Open Access *Correspondence: nhxia@scbg.ac.cn 1 Key Laboratory of Plant Resources, Conservation and Sustainable Utilization, Guangdong Provincial Key Laboratory of Digital Botanical Garden, South China Botanical Garden, Chinese Academy of Sciences, Guangzhou 510650, China Full list of author information is available at the end of the article in a few patches came to our attention. This species has leptomorph rhizomes, 3-5 branches per mid-culm node, just a single foliage leaf per ultimate branch, conventional spikelets, 3 stamens and 2 stigmas, which fall within the circumscription of Gelidocalamus. But after detailed examination of its vegetative and reproductive characters, we concluded that this bamboo is new to science, and is described and illustrated here. We also paid special attention to the development of the branch complement in relation to flowering.

Methods
Flowering material was dissected under a stereo microscope (Mshot-MZ101) and images were taken with the camera attachment (Mshot-MSX2). Morphological comparisons were based on relevant literature Lin 1988Lin , 1990Lin , 1992Nie et al. 2018;Wen 1982), specimens and living plants. Both the type specimen and photos were used for making descriptions. The terms applied to the flowering structure in the analysis mainly follow the synflorescence concept applied to grasses (Cai and Xia 2021;Muchut et al. 2018;Reinheimer and Vegetii 2008;Stapleton 1997;Tivano et al. 2009;Vegetti and Müller-Doblies 2004). As the feature of branches bearing only a single foliage leaf is uncommon among bamboos, we specially examined and documented branch complements and axes at different stages of flowering to adduce their development from onset of flowering to senescence. Diagnosis. G. fengkaiensis is similar to G. stellatus, but differs by its mucronate (versus aristulate) lemma apices, palea surfaces between keels that are pubescent (versus glabrous) at the upper half, acute (versus bifid) palea apices, 3-5 (versus 7-12) branches at mid-culm nodes, pubescent (versus glabrous) culm sheath ligules, culm sheaths with one margin densely ciliate and the other glabrous or apically ciliate (versus both margins glabrous), and larger foliage leaves of 15-29 × 4-6 cm (versus 12-17 × 1.3-2.2 cm) ( Table 1).
Distribution and habitat. This species is known only from the type locality, Qixingding Nature Reserve, Fengkai County, Guangdong Province, China. It commonly grows in secondary forest, under broad-leaved forest or by the roadside, at 500-600 m a.s.l. It is also found on the outskirts of villages.
Additional specimen examined (paratype

Branch development in relation to flowering
In Gelidocalamus fengkaiensis, the midculm branch complement (sensu McClure 1966) arises from a single primary bud. This bud produces a primary branch axis (subtended by a prophyll, Fig. 2F) that has a broad base but which soon continues to develop as a slender axis distally, at the same time developing several secondary branch axes from its basal nodes. This produces the basic branch complement of a cluster of several slender subequal ('initial') branches (i.e., of one primary and several secondary axes), with no obvious dominant member. Each of these 'initial' branches in the basic branch complement develops a few closely spaced nodes basally but more distantly spaced nodes distally, characteristically giving rise to 3-4 elongate internodes (the primary branch axis itself sometimes developing up to five elongate internodes). Most of the branch nodes have buds and a number of these buds develop into similar vegetative branches of progressively higher order in subsequent growing seasons (Fig. 3B).
Two branch types can be distinguished in G. fengkaiensis as well as the generic type, G. stellatus. We call the branches, each of which terminates in a single foliage leaf, 'foliage branches' (= branches related to the production of leaves) (Figs. 1A, B, 3A, B). The more distal internodes on such foliage branches all have sheaths that are significantly longer than those at the branch base, but these also only have rudimentary blades, not expanded into the green 'leaf blades' that are typical of bamboo foliage leaf complements. In other species of Gelidocalamus, the distal-most two or several sheaths on a branch may also develop expanded green leaf blades (Zhu and Stapleton 2006). In G. fengkaiensis, typically, each foliage branch axis has a series of several well-spaced nodes but with only the distal-most node bearing the solitary expanded green leaf blade (Figs. 1A and 3A, B) (very exceptionally, 2-more such green leaf blades are borne). The final sheath of such a branch does not encase any elongate branch portion within, as revealed by sections of the distal-most sheath, but the whole resembles a twig due to its rigidity (Fig. 1I, J).
We also noted, from sections of the terminal leaf sheaths of foliage branches, that these were medially very much thickened and tightly rolled around the terminal meristem. Thus, in Gelidocalamus, the terminal branch sheath of foliage branches elongates far beyond the apical meristem and is especially thickened, and ideally could be expected to afford effective insulation against damage or drying out of the apical meristem. In summary, the typical characteristics of G. fengkaiensis include a branch complement of several slender branches developed from a solitary primary branch bud; these initial branch axes individually bear only a terminal expanded foliage leaf blade (and are here referred to as foliage branches); and the distalmost leaf-sheath on such foliage branches far exceed the branch apex proper (i.e., apical meristem) and is especially thickened and tightly rolled, forming a rigid structure that supports the expanded terminal leaf blade.
The other branch type terminates in an inflorescence which we can refer to as a 'flowering branch' (Fig. 2A). As contrasted with foliage branches, flowering branches do not terminate in any expanded green leaf blades and appear to be specialized reproductive axes.
In the material of G. fengkaiensis we studied, the apical meristem (at the apex of the terminal branch internode) of a foliage branch appears to be hardly developed, even with the onset of flowering, so that this does not seem to elongate in time and appears to be dormant or even eventually senescent. We have found no evidence of such foliage branches (of primary or higher orders) continuing to develop a flowering axis. Apparently, during a flowering episode, flowering branch development is initiated by the available (axillary) buds at nodes on the primary and higher-order branch axes, i.e., buds that have not already developed earlier into foliage branches.
In G. fengkaiensis, we noted that whole-plant flowering seemed to be typical, but not all branch axes flowered simultaneously. Eventually, all branches, including both flowering branches or non-flowering foliage branches, became senescent and would perish. Flowering is supraannual rather than annual. This would partly account for why to-date, G. fengkaiensis is only the fourth species of the genus for which we know the flowering structure. A flowering episode should last several months at least: we received information that this species was blooming in April, but when we went to collect it in August, flowering was clearly dwindling down and the culms were already starting to perish. The whole plant would die following flowering, essentially representing a monocarpic life history.